It is often desirable to install apparatus for transmitting and receiving control and telemetry signals as an adjunct to existing equipment connected to a common source of power. Such apparatus enables a user in one location to transmit a signal to another location for purposes of controlling equipment thereat, or to receive information from a remote location concerning the status of equipment or other conditions thereat.
In known systems used for home security and lighting control purposes, for instance, coded signals are applied to an a.c. power main's outlet at one location, and are received at another location for controlling a device or for monitoring conditions at the first location. Typically, such signals are transmitted by means of a high-frequency carrier impressed upon the a.c. main's voltage, and modulated with a specific code designating the particular function desired, the receiver of such signals being adapted to detect the high-frequency signals and to demodulate them, to recover the specific code and act on it to perform the desired function, for example to switch a light on or off, or to display the condition present at the transmitter's location.
Although such systems have been applied extensively for home use, a similar system may be particularly desirable for automobile use, and because the power supply of an automobile or other vehicle employs direct current (d.c.), the transmitter and receiver may advantageously be made simpler than is possible for the known systems mentioned above. However, a d.c. power supply such as that used in a vehicle direct current power system usually has an extremely low internal resistance, on the order of approximately 1-2 milliohms. Thus, it is difficult to impress control signals on the lines of the system, and the use of such control signals will often interfere with other components of the system.
Known remote control systems for use in automobiles or other vehicles therefore typically employ either a radio frequency (r.f.), ultrasonic, or infrared (IR) transmission system having no direct wiring between transmitter and receiver-actuator modules, or employ direct additional wiring between these modules. These systems all have their disadvantages.
R.f. systems are limited to communications between modules in the same space in the car, as there is usually a metal bulkhead between the passenger and engine compartments which prevents reliable radio transmission and reception between them. A similar limitation occurs with infrared and ultrasonic transmissions. Direct wiring between the two areas, however, involves difficulties for user installation, as the bulkhead also forms a firewall and the seal must be effectively maintained.